1. Field
The present invention relates to a capacitance-type transducer. Specifically, the present invention relates to a capacitance-type transducer constituted by a capacitor structure made up of movable electrodes (diaphragm) and fixed electrodes.
2. Related Art
In recent years, there has been demand for microphones to detect sounds with high sensitivity in a range from low sound pressure to high sound pressure. Accordingly, if the harmonic distortion rate can be reduced, it is possible to raise the maximum input sound pressure and widen the detectable sound pressure range (referred to hereinafter as the “dynamic range”) of the microphone. However, in general microphones, there is a trade-off relationship between an improvement in the acoustic vibration detection sensitivity and a reduction in the harmonic distortion rate, and it has been difficult to provide a microphone with a wide dynamic range from low-volume (low sound pressure) sounds to high-volume (high sound pressure) sounds.
In view of these circumstances, Japanese Patent No. 5252104 (referred to hereinafter as “Patent Document 1”) discloses a microphone in which the diaphragm is divided into two so as to be constituted by a low-volume high-sensitivity sensing portion and a high-volume low-sensitivity sensing portion, in order to widen the dynamic range of the microphone.
FIG. 1A is a schematic diagram showing the structure of an acoustic sensor 11 used in the microphone disclosed in Patent Document 1. In this acoustic sensor 11, the diaphragm is divided into two diaphragms by a slit 17, one diaphragm (first diaphragm 12a) having a comparatively large surface area, and the other diaphragm (second diaphragm 12b) having a comparatively small surface area. A fixed electrode 13 opposes the first diaphragm 12a and the second diaphragm 12b across an air gap, and the fixed electrode 13 is held by a back plate 14. In this acoustic sensor 11, a low-volume high-sensitivity first sensing portion 15a is constituted by the first diaphragm 12a and the opposing portion of the fixed electrode 13, and a high-volume low-sensitivity second sensing portion 15b is constituted by the second diaphragm 12b and the opposing portion of the fixed electrode 13. The dynamic range of the microphone is widened by switching the output from the acoustic sensor 11 between output from the first sensing portion 15a and output from the second sensing portion 15b in accordance with the volume.
Japanese Patent No. 5252104 is an example of background art.
In a capacitance type of acoustic sensor, when the diaphragm bends a large amount and comes into contact with the back plate, it is possible for the diaphragm to become attached to the back plate and not return to its original state (this phenomenon will be called “sticking”). Examples of cases where the diaphragm bends a large amount and sticking occurs include the case where the diaphragm is subjected to a loud sound, the case where the diaphragm of the acoustic transducer is subjected to high compressed air pressure (wind pressure) due to being dropped in a drop resistance test, and the case where air is forcefully blown into the acoustic sensor.
If the diaphragm becomes stuck to the back plate, vibration of the diaphragm is hindered, and it becomes impossible for the acoustic sensor to detect acoustic vibration. For this reason, in the acoustic sensor 11 shown in FIG. 1A, many stoppers 16 having a uniform projection length are provided on the back plate 14, and sticking of the diaphragms 12a and 12b is prevented by the stoppers 16.
However, the length (projection length) of the stoppers 16 influences the resistance of the diaphragms 12a and 12b to damage (damage resistance) and the resistance to sticking (sticking resistance). This point will be described below with reference to FIGS. 1B and 1C.
If the stoppers 16 are short as shown in FIG. 1B, the gap between the diaphragms 12a and 12b and the stoppers 16 is wide when no deformation is occurring, and therefore the diaphragms 12a and 12b can undergo a large amount of deformation. Also, when the same pressure is applied, the first diaphragm 12a will undergo more deformation than the second diaphragm 12b. The first diaphragm 12a therefore more easily comes into contact with the stoppers 16, and more elastic restoration force is generated upon coming into contact with the stoppers 16, and therefore the first diaphragm 12a more easily detaches from the stoppers 16 (is less likely to become stuck). In contrast, the second diaphragm 12b has a comparatively smaller surface area and higher rigidity, and therefore normally does not undergo a large amount of deformation. However, it undergoes a large amount of deformation when subjected to a high pressure load in a drop resistance test or the like as described above. In this case, if the stoppers 16 are short, the gap between the second diaphragm 12b and the stoppers 16 is wide, and therefore the second diaphragm 12b undergoes a large amount of bending deformation before the deformation is stopped by coming into contact with the stoppers 16, and the second diaphragm 12b easily becomes damaged.
If the stoppers 16 are long as shown in FIG. 1C, the gap between the diaphragms 12a and 12b and the stoppers 16 is narrow when no deformation is occurring, and therefore deformation of the diaphragms 12a and 12b is suppressed by the stoppers 16. For this reason, even if a high pressure load is applied to the second diaphragm 12b, the second diaphragm 12b does not undergo a large amount of deformation, thus making it unlikely for the second diaphragm 12b to bend a large amount and become damaged. In contrast, if the stoppers 16 are long, the gap between the first diaphragm 12a and the stoppers 16 is narrow, and therefore there is a reduction in the amount of elastic restoration force generated when the first diaphragm 12a comes into contact with the stoppers 16, and the first diaphragm 12a does not easily detach from the stoppers 16, and is likely to become stuck.